Method and system for low voltage electrochemical gas to liquids production

ABSTRACT

A method for producing methanol from methane in which methane is provided to an anode electrode having a metal oxide catalyst disposed on an anode side of an electrolyte membrane, thereby producing methanol and electrons on the anode side. The electrons are conducted to a cathode electrode such as having an oxygen reduction catalyst disposed on a cathode side of the electrolyte membrane, thereby transforming oxygen and water provided to the cathode side to hydroxide ions.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional Patent Application, Ser. No. 62/082,751, filed on 21 Nov. 2014. The co-pending Provisional Application is hereby incorporated by reference herein in its entirety and is made a part hereof, including but not limited to those portions which specifically appear hereinafter.

The subject matter of this application is also related to prior U.S. patent application Ser. No. 13/670,501, filed on 7 Nov. 2012 (now U.S. Pat. No. 9,163,316, issued 20 Oct. 2015), Ser. No. 13/719,267, filed on 19 Dec. 2012, and Ser. No. 14/076,445, filed on 11 Nov. 2013. The disclosures of these related patent applications are hereby incorporated by reference herein and made a part hereof, including but not limited to those portions which specifically appear hereinafter.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under grant DE-AR0000307 awarded by the U.S. Department of Energy (DOE). The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to gas to liquids processing and, more particularly, to electrochemical processing of gas to liquids. In one aspect, this invention relates to a method for producing liquid organic fuels from hydrocarbons. In one aspect, this invention relates to an electrochemical method for producing liquid organic fuels from hydrocarbons. In one aspect, this invention relates to a system for the electrochemical production of liquid organic fuels from hydrocarbons.

2. Discussion of Related Art

Methane is an abundant hydrocarbon material especially with the development and production of shale gas. To date, however, methane has been generally underutilized as a precursor for chemicals and liquid fuels due to the difficulty of efficiently transporting methane (e.g., gas), particularly from remote or scattered sites. Methanol is one of the 25 top chemicals produced worldwide; it is the main feedstock for the chemical industry; and it is a source of dimethyl ether (DME), which could be used as a vehicular fuel.

There is presently no commercially available process or system for the economical, scalable, and portable conversion of methane to methanol or other gaseous alkanes to alcohols. In the past, Fischer-Tropsch processing has been applied to produce methanol via a Fischer-Tropsch reaction involving the high temperature steam reforming of methane followed by high pressure reaction of the reformate hydrogen and CO. The efficiency of such processing, however, is generally only about 50-65% depending on the waste heat recovery.

Thus, more efficient and cost effective conversion of methane to methanol is very much desired. Moreover, while Fischer-Tropsch processing can perform the ultimate conversion, such processing is generally only economical on very large scales and is not readily portable. It is generally not possible to economically perform Fischer-Tropsch conversion of methane to methanol or other alkanes to alcohols directly at natural gas wells or other scattered or smaller scale sites, including wastewater treatment plants and landfill gas sites, for example.

More specifically, there is a need and a demand for processing methods and systems applicable to or for the conversion of hydrocarbons, such as alkanes, for example, and particularly lower hydrocarbons, such as lower alkanes such as methane, for example, to more easily transportable and/or higher valued hydrocarbon materials such as alcohols, e.g., methanol.

As disclosed in above-identified and herein incorporated, U.S. patent application Ser. No. 13/670,501, filed on 7 Nov. 2012, one current electrochemical gas to liquids approach is based on the following reactions:

Anode:

Ni(OH)₂+OH⁻→NiO⁺OH⁻+H₂O+1e ⁻(−0.52V)  1)

Losing Electrons, Oxidation

NiO⁺OH⁻+CH₄→CH₃OH+OH⁻+Ni⁺  2)

Ni⁺+2OH⁻→Ni(OH)₂+1e ⁻  3)

Cathode:

2H₂O+2e ⁻→H₂+2OH⁻(−0.8277 V)  4)

Gaining Electrons, Reduction

The net reaction is:

CH₄+H₂O→CH₃OH+H₂

This reaction is based on the water activation of methane.

Total Cell Voltage: −1.3477V

A membrane is used to separate the anode and cathode electrodes. The membrane can be porous or have ionic conducting sites. The membrane is not electronically conductive. The membrane allows for OH⁻ ions to transport from the cathode to the anode.

The portion of the voltage required to split water at the cathode and generate hydrogen is (−0.8277 V/−1.3477 V)=61.4%. Though hydrogen can be valuable, it is here ultimately an undesirable byproduct as the generally sought objective is to convert gaseous methane to liquid methanol. Thus, generating gaseous hydrogen is somewhat contrary to the desired goal, and requires 61.4% of the voltage.

SUMMARY OF THE INVENTION

This invention relates generally to gas to liquids processing and, more particularly, to electrochemical processing of gas to liquids.

In accordance with one aspect, the invention improves upon presently used electrochemical gas to liquid reactions by replacing a cathode water splitting/hydrogen evolution reaction with an oxygen reduction reaction.

As described in greater detail, practice of the invention can significantly lower system voltage and power requirements.

In accordance with one aspect of the invention, there is provided a new method for converting methane to methanol. In one embodiment, such a method involves providing a reactor including an anode, a cathode, and a membrane between the anode and the cathode. Methane is fed to the anode and converted to form methanol and electrons. The electrons are passed to the cathode and water and oxygen are fed to the cathode, reducing the oxygen and producing hydroxide ions. The hydroxide ions are transported through the membrane to the anode and methanol is recovered from the reactor.

Thus, the reaction at the cathode is desirably an oxygen reduction reaction such as catalyzed via an oxygen reduction catalyst such as cobalt polypyrrole, silver on carbon, Pt, and/or MnO₂.

The feeding of water and oxygen to the cathode may desirably involve at least one of bubbling an oxygen-containing gas through water to the cathode and feeding a humidified oxygen-containing gas to the cathode.

In accordance with another aspect of the invention there is proved a system for converting methane to methanol. In accordance with one embodiment, such a system may desirably include:

a reactor having an anode, a cathode, and a membrane between the anode and the cathode; the membrane being at least one being porous or having ionic conducting sites, the membrane allowing transport of hydroxide ions from the cathode to the anode;

a source of methane interconnected to the anode to permit the feed of methane to the anode; and

a source of oxygen and a source of water interconnected to the cathode to permit the feed of water and oxygen to the cathode.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of this invention will be better understood from the following detailed description taken in conjunction with the drawings, wherein:

FIG. 1 is a diagrammatic representation of the technology for conversion of methane to methanol at low temperatures in accordance with one embodiment of the invention;

FIG. 2 is a diagrammatic representation of a membrane for use in one embodiment of a method of this invention and the reactions associated therewith;

FIG. 3 is a diagram showing a methane to methanol reactor for carrying out the method in accordance with one embodiment of this invention;

FIG. 4 is a graphical presentation of methanol concentration and applied voltage, respectively, versus time realized with a test system for converting methane to methanol in accordance with one embodiment of the invention.

DETAILED DESCRIPTION

As described in greater detail below, the invention generally relates to improved methods and systems for low voltage electrochemical gas to liquids production.

In accordance with one aspect of the invention, we have discovered that significantly improved desired performance can be realized by changing the cathode reaction from reaction #4 set forth above (i.e., 2H₂O+2e⁻→H₂+2OH⁻) to:

½O₂+H₂O+2e ⁻→2OH⁻(+0.401V)  5)

Gaining Electrons, Reduction

For example, with such change, the system voltage requirements can be significantly reduced.

Reaction #5 is a relatively well-known and established reaction sometimes commonly referred to as the “oxygen reduction reaction” (ORR). For example, the ORR is a common cathode reaction for alkaline fuel cells and alkaline metal air batteries. A significant benefit of the ORR in an alkaline environment is that precious metal catalysts such as platinum are not required. Prior experience with reaction #5 in or for the cathode of alkaline fuel cells, has included using cobalt polypyrrole, silver on carbon, Pt, MnO₂, or other possible cathode catalysts, for example.

Since the voltage for reaction #5 is positive, the total cell voltage combined with reaction 1 is now:

(−0.52 V)+(+0.401 V)=(−0.119 V)

The required voltage of −0.119V is only 8.83% of the required voltage for reactions #1-4, representing a voltage savings of 91%. The required current is the same for both sets of reactions, i.e., reactions #1-4 and the subject reactions #1-3 and #5. However, since power=voltage×current, the required power savings realizable through the practice of the invention will be as high as 91%.

With the use of reaction #5 in place of reaction #4, the whole overall reaction is based on the oxygen activation of methane and becomes:

2CH₄+O₂→2CH₃OH

The primary changes to the system necessitated by the use of reaction #5 in place of reaction #4 include:

1) change of the cathode catalyst to a catalyst which can perform the ORR, e.g., cobalt polypyrrole or silver on carbon, and

2) provide oxygen to the cathode such as by bubbling air to the cathode of the gas to liquids reactor or feeding humidified air to the cathode, for example.

Many other catalyst materials can be used including, for example, conventional Pt or Pt on carbon catalyst.

As noted above, a system in accordance with one aspect of the invention can operate at a significantly lower voltage of around −0.119V per cell. An additional benefit is that the amount of water consumed by the cathode is equal to the amount of water generated on the anode. As may be desired, the generated water can be appropriately separated from the methanol product and recycled back to the cathode. Thus, theoretically in one preferred practice of the invention, no external water input is necessary if all water was recovered from methanol/water separation.

Previous electrochemical gas to liquids processing via above reactions #1-4 produces hydrogen and methanol. Both are valuable products and system economics can work out favorably provided a source of electricity is readily available and there is a need for the generated hydrogen. Our models, however, show that when such a system is scaled to a large size for field use in converting natural gas at a well head at a flow rate of 1,000,000 standard cubic feet per day, the power requirement is quite large around 3600 kW. Typically gas wells are found in remote locations where grid power is not readily available. Additionally the hydrogen gas formed by such a system may be of little to no use in such remote locations. Moreover, the need to generate the required power at the remote site can and typically will involve a significant initial capital cost. By replacing the water splitting/hydrogen generating reaction on the cathode with an oxygen reduction reaction, such as through a preferred practice of one aspect of the invention, the voltage requirement applied to the cell can be significantly decreased or reduced. For example, in one embodiment, the voltage requirement applied to the cell can be decreased from 1.35V to 0.12V. Therefore the power requirement is approximately 1/10^(th) of that required before, or about 360 kW which is significantly much more manageable and greatly improves the operating profitability such as due to reduced electricity costs.

A major advantage of processing and systems such as herein described include increased portability, high scalability and facilitation of operation at low temperature (e.g., operation at temperature of less than 200° C.). Moreover, compared to the existing electrochemical gas to liquids processing, preferred practice of the invention desirably significantly reduces electrical consumption, minimizes or avoids formation or production of hydrogen, and has theoretically zero net water consumption.

The method of this invention can desirably utilize inorganic metal oxide cation intermediates as catalysts to oxidize methane to methanol at temperatures less than 200° C. and, in some embodiments at a temperature less than or equal to about 160° C., preferably, at room temperature. The metal oxide cation intermediates, which, upon reaction, are transformed to a noncatalytic form, are regenerated electrochemically at the anode in a battery-type reactor with hydrogen production at the cathode. Thus, this method produces methanol from methane and water at room temperature with high efficiency and high selectivity without using a high temperature Fischer-Tropsch process. As used herein, the term “high efficiency” refers to efficiencies greater than about 80%, and the term “high selectivity” refers to selectivities greater than about 90%.

Accordingly, the methane to methanol process of this invention applies an electrochemical process to continuously maintain the catalytic property of the metal oxide anode. In this process, methane is fed to the anode, producing methanol, water and electrons. The electrons are conducted to the cathode where they transform oxygen and water provided to the cathode to hydroxide ions. The hydroxide ions are transferred through the membrane separator disposed between the anode and cathode electrodes to the anode for regeneration of the oxidation metal oxide cation catalyst. The process is continuous as long as sufficient electrical current is applied. Although regeneration of an anode oxidation catalyst is a known technology practiced in batteries, such technology as practiced is unsuitable for methane to methanol conversion because the anode compositional and structural engineering and design are unsuitable for this purpose.

FIG. 1 is a schematic diagram showing the methane to methanol process in accordance with one embodiment of this invention in an exemplary electrochemical cell comprising an electrolyte membrane 10 disposed between an anode and cathode, a metal oxide catalyst 11 disposed on the anode side of the electrolyte membrane and a suitable oxygen reduction catalyst 12, such as described above, disposed on the cathode side of the electrolyte membrane.

The system may be in an alkaline environment or an acidic environment. In an alkaline system in accordance with one embodiment of this invention, the electrolyte membrane comprises an alkaline electrolyte. In accordance with one embodiment, the electrolyte membrane is a porous polymer layer containing an alkaline electrolyte. An alkaline electrolyte is preferred because, in an alkaline solution, the oxygen overpotential is much lower than that in an acidic solution. In an acidic solution, a proton exchange membrane may be used.

As shown in FIG. 2, the product at the cathode is hydroxide ion, which is transported through the membrane to the anode where it reacts with nickel cations to form nickel oxide hydroxide, water, and electrons.

FIG. 3 shows a gas-to-liquid reactor for use in accordance with one embodiment of the method of this invention. The reactor as shown comprises two cells. Each cell comprises a membrane separator 20 disposed between gas diffusion electrodes 21. In accordance with one preferred embodiment of this invention, the membrane separator has a thickness in a range of about 20-50 μm and the gas diffusion electrodes have a thickness in a range of about 200-300 μm. The cathode electrode comprises an oxygen reduction catalyst layer 22 and the anode electrode comprises a NiOOH catalyst layer 23. A bipolar plate 24 separates the individual cells from one another and is configured to provide flow channels 26. The apparatus further comprises a current collector 25 disposed between the bipolar plate 24 and the end plate 27. A power supply 30 provides an electrical current to the cells. This setup is a combination of fuel cell technology and nickel metal hydride battery technology for which all of the materials are commercially available.

The present invention is described in further detail in connection with the following examples which illustrate or simulate various aspects involved in the practice of the invention. It is to be understood that all changes that come within the spirit of the invention are desired to be protected and thus the invention is not to be construed as limited by these examples.

Examples

In this example, a system for converting methane to methanol in accordance with one embodiment of the invention and operated in the manner detailed below, produced or generated the results shown in FIG. 4.

System:

Anode: Ni(OH)₂, Vulcan XC72, 16 mg/cm², 10% PTFE, 2% Nafion, carbon paper

Membrane: Microporous polyethylene

Cathode: 5 mg/cm² Ag/C on carbon cloth and 6M Liquid potassium hydroxide

Operation:

Heated to 80° C., Anode fed pure Methane, and Cathode fed Air.

Methane was fed to the anode at 2 ml/min and the anode exhaust was monitored using gas chromatography. Air was bubbled through a cathode containing 6M potassium hydroxide. With cell voltage of 0.2V the methanol concentration in the exhaust was about 50 ppm. When the voltage was increased to 0.35V the methanol concentration in the exhaust increased to over 100 ppm. After 45 hours the anode was rinsed with DI water and dried with nitrogen. Upon restarting the experiment at 0.35V applied potential, the methanol concentration spiked to 4466 ppm before decreasing to 1250 ppm then gradually decreasing to 50 ppm.

From this experiment it is concluded that the system produces methanol from methane under an applied voltage. It is preferred that the anode catalyst remain dry so the transport of methane to the catalyst surface is not inhibited.

The invention illustratively disclosed herein suitably may be practiced in the absence of any element, part, step, component, or ingredient which is not specifically disclosed herein.

While in the foregoing detailed description this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purposes of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention. 

What is claimed is:
 1. A method for converting methane to methanol, the method comprising: providing a reactor including an anode, a cathode, and a membrane between the anode and the cathode; feeding methane to the anode, and converting the methane to form methanol and electrons; passing the electrons to the cathode and feeding water and oxygen to the cathode, reducing the oxygen and producing hydroxide ions; transporting the hydroxide ions through the membrane to the anode; and recovering the methanol from the reactor.
 2. The method of claim 1 wherein reaction at the cathode is an oxygen reduction reaction via an oxygen reduction catalyst.
 3. The method of claim 2 wherein said oxygen reduction catalyst is selected from the group consisting of cobalt polypyrrole, silver on carbon, Pt, and MnO₂.
 4. The method of claim 1 wherein said feeding of water and oxygen to the cathode comprises at least one of bubbling an oxygen-containing gas through water to the cathode and feeding a humidified oxygen-containing gas to the cathode.
 5. The method of claim 4 wherein said feeding of water and oxygen to the cathode comprises bubbling an oxygen-containing gas through water to the cathode.
 6. The method of claim 5 wherein the oxygen-containing gas comprises air.
 7. The method of claim 5 wherein the oxygen-containing gas comprises O₂.
 8. The method of claim 4 wherein said feeding of water and oxygen to the cathode comprises feeding a humidified oxygen-containing gas to the cathode.
 9. The method of claim 8 wherein the oxygen-containing gas comprises air.
 10. The method of claim 8 wherein the oxygen-containing gas comprises O₂.
 11. The method of claim 1 comprising performing the following net chemical reaction at the cathode: ½O₂+H₂O+2e ⁻→2OH⁻.
 12. The method of claim 1 comprising performing the following overall reactor reaction: 2CH₄+O₂→2CH₃OH.
 13. A system for converting methane to methanol said system comprising: a reactor having an anode, a cathode, and a membrane between the anode and the cathode; the membrane being at least one being porous or having ionic conducting sites, the membrane allowing transport of hydroxide ions from the cathode to the anode; a source of methane interconnected to the anode to permit the feed of methane to the anode; and a source of oxygen and a source of water interconnected to the cathode to permit the feed of water and oxygen to the cathode.
 14. The system of claim 13 wherein the cathode includes an oxygen reduction catalyst.
 15. The system of claim 14 wherein the oxygen reduction catalyst is selected from the group consisting of cobalt polypyrrole, silver on carbon, Pt, and MnO₂.
 16. The system of claim 13 wherein the source of oxygen and the source of water interconnected to the cathode comprises one of bubbling air through water to the cathode and feeding humidified air to the cathode.
 17. The system of claim 16 wherein the source of oxygen and the source of water interconnected to the cathode comprises bubbling air to the cathode.
 18. The system of claim 16 wherein the source of oxygen and the source of water interconnected to the cathode comprises feeding humidified air to the cathode. 